Organic tin derivatives, in particular dialkyltin oxides, such as dibutyltin oxide (Bu2Sn═O, referred to hereinafter as DBTO), are widely described in the literature as transesterification catalysts for the synthesis of (meth)acrylic esters.
DBTO is provided in the form of a white powder which is insoluble in water and virtually insoluble in alcohols, such as methanol or ethanol, or aminoalcohols, in hydrocarbon solvents and in (meth)acrylic esters, such as methyl acrylate or dimethylaminoethyl acrylate. It dissolves slowly in alkaline solutions; it is dissolved in carboxylic acids, with which it forms carboxylated derivatives.
DBTO in the solid form is widely used in transesterification processes of batch type. The use thereof is relatively simple in this case, although, however, it has to be handled with care due to its pulverulent state, which can result in risks of inhalation of a product exhibiting a toxicity related to that of tin.
The use of DBTO in the solid form proves to be more difficult in the case of a continuous transesterification process or when it is necessary to introduce DBTO continuously. Solutions exist, for example for the use of a system for introducing the powder via an endless screw, but these solutions are very awkward and do not obviate risks of inhalation of fine particles of tin-based compound.
The Applicant Company has found a simple and original means for using DBTO in the form of a concentrated solution. For this reason, it becomes easy to continuously introduce it in the form of a solution using a simple pump with all the advantages which this represents in terms of ease of processing and of health.
The use of organic tin derivatives, in particular DBTO and its homologs, in the synthesis of (meth)acrylic esters is widely described in the literature.
Mention may be made of the patent application JP 56-104851, which describes the use of a tin oxide, in particular DBTO, as transesterification catalyst in the synthesis of tert-butylaminoethyl methacrylate from methyl methacrylate and tert-butylaminoethanol.
The U.S. Pat. No. 3,642,877 describes the use of DBTO as catalyst in the synthesis of dimethylaminoethyl methacrylate by transesterification between methyl methacrylate and N,N-dimethylaminoethanol. DBTO is used in the solid form and it is introduced into the reaction medium only after an azeotropic distillation of the water with the methyl methacrylate has been carried out in order to prevent any deactivation of the DBTO by the water present in the reactants.
The patent application JP-A1-3-118,352 describes a process for the synthesis of dialkylaminoalkyl esters of (meth)acrylic acid by transesterification in a solvent which is inert to the reaction, such as hexane, under a pressure of 1.5 to 3 atmospheres, in the presence of powdered DBTO as catalyst.
Provision has been made, in the patent application US 2006/0173191, to introduce the transesterification catalyst, such as DBTO, in the form of several charges into the reaction reactor, the catalyst being introduced in the solid form, or as a mixture with crude reaction product withdrawn from the reactor, or else in the form of a suspension in methyl methacrylate. Under these conditions, the use of DBTO in concentrated homogeneous solution is not suggested.
In the patent application JP 2001-187763, the (meth)acrylic acid corresponding to the targeted (meth)acrylic ester is used to dissolve a dialkyltin oxide before being introduced in the solution form as transesterification catalyst, which makes it possible to avoid any problem of reprecipitation or of deposition of solid when the reaction temperature decreases.
The U.S. Pat. No. 7,078,560 claims the use of organic tin derivatives in the form of distannoxanes as transesterification catalysts in the synthesis of aminoalkyl (meth)acrylates for overcoming the problem of insolubility of DBTO in N,N-dimethylaminoethanol and more generally in aminoalcohols. The distannoxane is obtained by reaction of an organic tin oxide with its corresponding tin halide. By way of example, octabutyltetrachlorodistannoxane is prepared from DBTO and dibutyltin dichloride (Bu2SnCl2). This distannoxane can be dissolved at a level of 10% by weight or more in N,N-dimethylaminoethanol. According to this method, DBTO, in order to be dissolved in N,N-dimethylaminoethanol, has to be converted beforehand into another chemical entity, namely a distannoxane, which results in a complex preparation for the catalyst.
The Applicant Company has found a simpler means targeted at dissolving dibutyltin oxide at relatively concentrated contents, so as to generate catalytic solutions capable of being used directly in transesterification reactions.
Surprisingly, while DBTO is insoluble in alcohols and esters, it has been found that dibutyltin oxide dissolves in an alcohol/(meth)acrylic ester(s) mixture. This is the case, for example, for a mixture of alcohol and of methyl or ethyl (meth)acrylate, taken alone or in the presence of the heavy ester resulting from the transesterification reaction between the methyl or ethyl (meth)acrylate and the alcohol.
It is thus an aim of the present invention to provide a concentrated solution of dialkyltin oxide, more particularly of DBTO, which is homogeneous and stable over time at ambient temperature and which meets the health and safety requirements of legislation for the industrial sphere.
The aim of the present invention is also to provide a catalytic composition comprising DBTO or a homolog which can be used for the synthesis of (meth)acrylic esters by transesterification, which composition is prepared beforehand with pure compounds which can be employed in this synthesis, exhibiting an ease of use in a process of continuous type or when introduction of the catalyst continuously is necessary, and resulting in an improved selectivity with respect to solid DBTO or its homolog.